Use post type label for "Save Order" button

To improve compatibility with custom order types
Display order filters for custom order types

That declare they are using order-meta-boxes (and therefore, are assumed
to be exposing the Edit Order interface and a list table)
Change insert button label for custom order types
Apply request filters for custom order types
Delete order items for custom order types
Make no order notes label more generic

To account for it being displayed on custom order type Add/Edit screens
where the order type is not referred to as an 'Order' (e.g. a 'Subscription',
'Refund' or 'Pre-order').
Use order type's singular label for meta box names
Give child class access to billing/shipping fields

So that custom order types can extend `WC_Meta_Box_Order_Data` for their
own purposes without having to redefine all the billing/shipping fields
Use wc_get_order() not new WC_Order()

Because an order might be a custom order type
Merge pull request #6848 from thenbrent/custom-order-tweaks-2-2

Custom order type tweaks
[API] Prevented errors with deleted products in the orders endpoint, closes #6813
Changelog for orders API fix